1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to catadioptric optical systems used for microscopic imaging, inspection, and lithography applications.
2. Description of the Related Art
Currently available optical systems have the ability to inspect or image features on the surface of a specimen, such as inspecting defects on a semiconductor wafer or photomask, or alternately examining a biological specimen on a slide. Microscopes have been used in various imaging situations, including biology, metrology, semiconductor inspection, and other complex inspection applications where high resolution images of small areas and/or features are desired.
Typically available imaging systems include microscopes, which offer inspection using dry imaging, or imaging in a gaseous medium such as air. Certain newer applications may benefit from immersion imaging. Unlike dry imaging, immersion imaging immerses the sample in water or other liquid and images or inspects the image within the liquid. Immersion imaging can, in certain circumstances, provide increased optical properties, including but not limited to enhanced resolution, over dry imaging. Furthermore, due to the nature and fragile properties associated with certain biological specimens, biological imaging systems frequently cannot use dry imaging whatsoever. In this situation, a biological imaging system can only image a sample while the sample is immersed in liquid, necessitating the use of a system able to perform immersion imaging.
Problems with immersion imaging and objectives employed within immersion imaging systems include the ability to resolve the image using immersion imaging operation in the presence of either low wavelength or broad wavelength range light energy, or in the presence of different types of illumination and imaging modes. Further, immersion techniques cannot be universally employed with objectives available in standard equipment, such as microscopes.
Microscopes designed to support immersion imaging can be difficult to design for high resolution specimen imaging. To improve the quality of the image received, such a system may use various imaging modes to enhance the appearance of desired features on the specimen. Imaging modes used in an immersion imaging system may include bright field, dark field, differential interference contrast, confocal, and other imaging modes offering different benefits depending on the type of specimen, the features on the specimen being observed, the imaging environment, and other related considerations. Simply employing an immersion substance with an existing objective can, in many cases, provide less than acceptable images or results.
Certain imaging modes may employ light energy of varying wavelengths, and thus the ability to effectively resolve images and operate in the presence of a wide variety of wavelengths and over various wavelength ranges is particularly beneficial. In addition, many applications also require imaging over a large area while maintaining high resolution. Currently available immersion objectives do not allow imaging over a large area, also known as providing a large field size. For immersion imaging, the system objective is one of the most critical components of the design and may use light having a broad range of wavelengths. Currently available immersion objectives do not provide accurate imaging performance for light with a broad range of wavelengths. Further, with typical microscope designs and objective dimensions, it is unheard of to offer a device that provides a numerical aperture (NA) of 1.2, a field size of up to and in excess of approximately 0.250 millimeters, and that exhibits good visual performance.
The ability for an objective to operate within a standard microscope is both desirable and difficult to achieve for the detailed inspection performance required for semiconductor and biological inspections discussed above. No immersion objectives are currently known that can support broad wavelengths and exhibit large field sizes. Some dry objective designs may be highly corrected for broad band imaging at wavelengths, but they are typically incapable of high performance and having NAs above 0.95
It would therefore be beneficial to provide a system and objective for use in conjunction with standard microscopes and microscopy applications that overcome the foregoing drawbacks present in previously known dry imaging/immersion imaging systems. Further, it would be beneficial to provide an optical inspection system design having improved functionality over devices exhibiting the negative aspects described herein.